1. Field of the Invention
The present invention relates to an image forming apparatus and more particularly to an image forming apparatus for recording input images on a recording medium and for outputting them.
2. Related Background Art
In an image forming apparatus for forming images using the electrophotography system, the control of image forming conditions such as development contrast electric potential, fogging prevention electric potential have been controlled by adjusting the charge electric potential of a photosensitive drum and developing bias electric potential to be applied to a developing device, and the like to keep the density of the images formed constant. Especially, in multi-color copiers, unless the image forming conditions are set in accordance with the characteristics of the developer for each color, the density will vary from color to color resulting in an image which can not be considered good as a whole. It is therefore indispensable to keep the density constant for each color component. However, there has been a problem that since the variation in image density due to changes in humidity is especially significant and the degree of the density variation resulting from the moisture absorption varies depending on the colors, i.e. types of developers, the difference in density is striking.
The methods being contemplated to cope with such a problem include a method wherein a sensor is provided for measuring environmental changes, i.e., humidity in the vicinity of a developing device to obtain the quantity of the moisture absorbed by the developing device, and image forming conditions are changed by a control means in accordance with the result detected by the sensor in a predetermined period of time and the type of the developer used, and a method wherein a control means is provided, which chronologically changes set image forming conditions on the basis of the quantity and direction of the change in the result detected by an environment detecting means in a unit period of time. However, in order to improve the accuracy of the control, the control must be performed with complicated and expensive apparatus which can be a problem.
Also, a number of means for representing half-tone images have been proposed. The applicant has proposed the pulse width modulation system as a method for improving contrast while maintaining high resolution with a relatively simple configuration (U.S. Pat. No. 4,800,442). According to this method, the formation of digital images using a laser beam printer or the like through the conversion of digital image signals into binary signals is carried out in a manner wherein the digital image signals are converted into analog signals to obtain the contrast for half-tone, and the analog signals are compared with periodic pattern signals such as triangular pulses to generate pulse-width-modulated binary signals which are used as signals for driving the laser beam source. FIG. 31 shows examples of the levels of the analog signals and triangular pulses and an example of the binary signal resulting from the pulse width modulation.
Thus, pulse width modulation on digital image signals provides both high resolution and high contrast at the same time. Of course, when it is attempted to reproduce contrast which is faithful to the original in the course of such conversion, corrections must be made in consideration of the input characteristics of the original-reading system or the output density characteristics of the printer.
FIG. 32 is a block diagram showing major parts of an image forming apparatus including a .gamma.-correction (contrast correction) function utilizing said pulse width modulation system.
Optical image information reflected from an original (not shown) is converted into analog electric signals by a CCD 111. The analog electric signals output by the CCD 111 are amplified by an amplifier 112 to an appropriate level and are converted into digital signals by A/D converter 113. The digital signals are subjected to corrections performed by a contrast (.gamma.) corrector 114 on variations in contrast occurring in areas such as the area between the input of images and the output of the images. In general, the use of a ROM or the like wherein contrast correction coefficients for the entire system are stored allows a particular input contrast signal to be converted into a contrast signal on which predetermined corrections have been made referring to a look-up table showing contrast correction coefficients at that time. Digital image signals thus corrected are re-converted into analog signals by a D/A converter 115 and are compared with triangular pulse signals obtained by a triangular pulse generating circuit 117. Reference numeral 116 represents a comparator for this purpose whose outputs ultimately become binary image signals which have been pulse-width-modulated depending on density. The binary image signals are output to a printer 118 and are used for controlling the switching on/off of a laser beam to output images in half-tone representation. That is, half-tone images are formed by controlling the emission period of a laser.
The major factors that determine .gamma.-characteristics are input characteristics (i.e. the characteristics of the conversion into analog electric signals performed by the CCD 111) and output characteristics (i.e. the density characteristics of the formation of images to be utilimately recorded on the basis of the quantity of the pulse width modulation at the printer 118). The CCD 111 generally has characteristics such that it exhibits monotonous change relative to the quantity of light while the characteristics of the printer 118 vary depending on the types of the devices.
FIG. 33 shows examples of the input characteristics and the typical input/output characteristics in the case that an electrophotography type laser beam printer is used, along with an example of the .gamma.-correction table in this case.
The density of the CCD 111 has the characteristics shown in the quadrant I in FIG. 33 relative to the density shown in the quadrant II. In order to obtain output characteristics faithful to input density, the value of the density input by the CCD 111 must be .gamma.-corrected as shown in the quadrant III. Therefore, it will be understood that the correction table shown in the quadrant IV may be provided on the contrast (.gamma.) corrector 114 shown in FIG. 32.
However, changes in the temperature and humidity in and around the printer result in changes in various characteristics such as latent image characteristics, developing characteristics and transfer characteristics. This causes the output characteristics shown in the quadrant III in FIG. 33 to change.
As a result, the output density relative to input density shown in the quadrant II changes. Consequently, the output image is formed in different ways from the same original depending on the changes in temperature and humidity.
In the proposal according to the application of the applicant, attempts have been made to cope with such a situation by, for example, employing a configuration wherein a temperature/humidity detecting means for detecting temperature and humidity is provided in the apparatus to correct the image data received by the main body of the image forming apparatus in accordance with the output signals from the temperature/humidity detecting means.
In the conventional configuration as described above, however, since the output image data corresponding to the input image data is developed into a table, an enormous amount of information is involved and, as a result, there is a tendency that a memory of large capacity is required.
Especially, when an electrophotography type laser beam printer as described above is used, there are factors affecting the printer conversion characteristics shown in the quadrant III in FIG. 33 other than the changes in the temperature and humidity in and around the printer as described above, the factors including the density of toner (or the quantity of the toner left) in the developing device, the number of sheets that the apparatus can endure, and the type of the transfer material used. Since a memory having a still larger capacity is required to cover all of these correction factors, a problem arises in that the apparatus inevitably becomes large and expensive.
In addition, alterations to the contents of a correction require the information in the table to be rewritten item by item. The labor for this tends to become more complicated as the amount of information increases.